1. FIELD OF THE INVENTION
The invention concerns the upward extrusion of polystyrene foam into a cooling media of boiling water. More specifically this invention deals with a means to form a density gradient from high density interior and exterior surfaces of a tubularly shaped extrudate to a low density center core of the extrudate, thereby increasing the strength and rigidity of the polystyrene foam product.
2. DESCRIPTION OF THE PRIOR ART
It has been long recognized that the mechanical properties of certain organic polymers, such as synthetic long chain polymers, can be substantially improved by subjecting such polymeric material to a drawing operation, at suitable temperatures, whereby the molecules of the polymer are oriented in the direction or directions in which the elongation is applied.
It has been previously proposed to produce molecularly oriented sheet material by extruding a hot ribbon of thermoplastic polymer and stretching the extruded ribbon simultaneously or sequentially, in both the longitudinal and the transverse directions. See for example U.S. Pat. Nos. 3,055,048 (Koppehele) and 3,676,539 (Fisher). Conventionally, biaxially orienting plastic film has involved the engagement of the edges of the sheet material by stretching devices which are intended to stretch the sheets in the two directions. However, such devices are somewhat less desirable when utilized to stretch normally brittle thermoplastic material such as polystyrene and polystyrene foam. Polymers of polystyrene are quite brittle at room temperature, and may fracture when subjected to an external force. In stretching polystyrene sheets, the edges of the thermoplastic material are engaged by gripping means which secure the material during the stretching process. The localized stresses created at the point of engagement, particularly where the gripping means are at a temperature lower than that of the thermoplastic material, are often sufficient to produce fractures of the material. Such fractures are particularly prevelant where the thermoplastic material is brittle such as is the case with polystyrene foam.
Work in recent years has accordingly been performed in the development of methods whereby the mechanical properties of thermoplastic materials such as polystyrene foam might be improved through biaxial orientation without the use of such stretching devices. U.S. Pat. No. 2,987,776 (Miller) in an example of such a process. That patent discloses a method whereby a liquid and gas mixture is injected into the hollow interior of a tubular film extrudate as it is being externally cooled. The mechanical properties of such a thermoplastic material may be additionally toughened by processes which further improve upon the strength increase derived from orienting the thermoplastic material molecules.
U.S. Pat. No. 3,299,192 (Lux) states that the rigidity, liquid handling, and thermal insulation capability of foamed plastic pipe was enhanced by quench chilling the internal and exterior walls of the tube within a short time after it emerged from an extrusion die. The patent notes that such chilling produces an impervious and non-porous (so as to promote liquid handling) internal and external skin on the pipe. The patent notes that the inner skin might be made 2 to 5 times as thick as the outer skin for purposes of carrying liquids such as water. The disclosure suggested coolant temperatures of 0 to 80.degree. F and velocities of 50 to 100ft/sec. The examples recite the use of 70.degree. F air as a coolant, blasted from a nozzle upon the extrudate surfaces. Such chilling produced a pipe having a core representing 50 to 94% of the thickness of the pipe, the interior and exterior skins representing the balance of the pipe thickness.
The present invention while related to the Miller and Lux patents distinguishes from them in its ability to very precisely control the cooling conditions of a particular thermoplastic, polystyrene foam, so as to greatly enhance that materials' strength properties. The present invention utilizes the unusual properties of a particular coolant to produce a unique polystyrene foam extrudate having a density gradient of decreasing value from the outside surface to the core.
It is an object of this invention to provide a means for the production of a polystyrene foam extrudate wherein the temperature of the cooling fluid, and thus the cooled extrudate may be very precisely controlled.
A further object of this invention is to provide means capable of precisely controlling a temperature gradient across a thickness of polystyrene foam extrudate.
An object of this invention is to cool from both the interior and exterior surface a polystyrene foam extrudate at a precise temperature selected from the range of about 15.degree. F to 55.degree. F above its glass transition point.
Another object of this invention is to inhibit polystyrene foam cell growth in such a manner as to produce an extrudate whose density and corresponding strength across an extrudate cross section are proportional to the tension and compression stresses which would be produced across such an extrudate cross section when such an extrudate has a moment applied at its end point.
A further object of this invention is to provide means capable of varying the degree of change (or slope) in density across a polystyrene foam extrudate.
Another object of this invention is to provide means to vary the time a polystyrene extrudate is cooled in a coolant bath without varying the rate at which polystyrene is extruded from an extension die.